On Thu, 8 Nov 2012 12:08:49 +0100, Lukas Czerner <lczerner@xxxxxxxxxx> wrote: > Currently ext4_ext_walk_space() only takes i_data_sem for read when > searching for the extent at given block with ext4_ext_find_extent(). > Then it drops the lock and the extent tree can be changed at will. > However later on we're searching for the 'next' extent, but the extent > tree might already have changed, so the information might not be > accurate. > > In fact we can hit BUG_ON(end <= start) if the extent got inserted into > the tree after the one we found and before the block we were searching > for. This has been reproduced by running xfstests 225 in loop on s390x > architecture, but theoretically we could hit this on any other > architecture as well, but probably not as often. > > ext4_ext_walk_space() is currently only used from ext4_fiemap() and even > if we do not hit the BUG_ON() fiemap might return scrambled information > to the user. > > Fix this by requiring ext4_ext_walk_space() to be called with i_data_sem > held. By calling it from ext4_fiemap() we can only take the i_data_sem > for read, but possibly other users might want to modify the extents so > they will be able to take write lock. Agree as a short term fix for BUGON case, but Theodore suggested to use seqlock approach http://lists.openwall.net/linux-ext4/2011/10/26/25 > > Signed-off-by: Lukas Czerner <lczerner@xxxxxxxxxx> > --- > fs/ext4/extents.c | 9 +++++++-- > 1 files changed, 7 insertions(+), 2 deletions(-) > > diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c > index 7011ac9..f1aca06 100644 > --- a/fs/ext4/extents.c > +++ b/fs/ext4/extents.c > @@ -1959,6 +1959,11 @@ cleanup: > return err; > } > > +/* > + * ext4_ext_walk_space() should be called with i_data_sem locked. If we're > + * not modifying found extents, or extent tree in callback function, then > + * read lock is ok. > + */ > static int ext4_ext_walk_space(struct inode *inode, ext4_lblk_t block, > ext4_lblk_t num, ext_prepare_callback func, > void *cbdata) > @@ -1976,9 +1981,7 @@ static int ext4_ext_walk_space(struct inode *inode, ext4_lblk_t block, > while (block < last && block != EXT_MAX_BLOCKS) { > num = last - block; > /* find extent for this block */ > - down_read(&EXT4_I(inode)->i_data_sem); > path = ext4_ext_find_extent(inode, block, path); > - up_read(&EXT4_I(inode)->i_data_sem); > if (IS_ERR(path)) { > err = PTR_ERR(path); > path = NULL; > @@ -5021,8 +5024,10 @@ int ext4_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo, > * Walk the extent tree gathering extent information. > * ext4_ext_fiemap_cb will push extents back to user. > */ > + down_read(&EXT4_I(inode)->i_data_sem); > error = ext4_ext_walk_space(inode, start_blk, len_blks, > ext4_ext_fiemap_cb, fieinfo); > + up_read(&EXT4_I(inode)->i_data_sem); > } > > return error; > -- > 1.7.7.6 > > -- > To unsubscribe from this list: send the line "unsubscribe linux-ext4" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
